U.S. Pat. No. 5,796,841, issued to Cordery, et al. on Aug. 18, 1998, (hereinafter referred to as the '841 patent) discloses a certificate meter. The certificate meter of the '841 patent is used in electronic commerce to account for a service charge associated with each use of the certificate meter and to ensure that upon receipt of a message the recipient can verify that (1) the message is genuine and signed by the sender (authentication) and (2) the message has not been altered (integrity). However, the period for which the certificate issued by the certificate meter is valid, from a security viewpoint, is dependent upon advances made in cryptoanalysis and computing power. That is, it should be assumed that the private key used to digitally sign the message will likely, at sometime in the future, be capable of being compromised. Accordingly, the period of time for which a signed message is considered to be valid is at least partially dependent upon the length of the private key used to sign the message. The larger the private key that is used, the more time consuming and complex are the computations required to compromise the private key.
In view of the above, one way to make the signed message more secure is to use to a private key that is extremely large. Thus, the private key can be made large enough so that any foreseeable advances in computing power will still make determination of the private key impractical. Unfortunately, as the size of the key increases the amount of processing time required to generate and verify a digitally signed message also significantly increases. The potentially large increase in processing time is not acceptable because it decreases the overall efficiency of the certificate meter system.
In addition to the above, not all messages require the same level of security. Some messages need to be protected for a significantly longer period of time and have a large value associated with them (e.g. a home mortgage contract). Other messages need to be protected for only a few years and have comparatively little value associated with them (e.g. a college ID). Still other messages occur on a frequent basis and therefore the time required to process them must be kept to a minimum (e.g. credit card transaction). As mentioned above, the additional processing overhead required to provide security for a long period of time is burdensome and unwarranted for messages that have only a short life and must be processed quickly. Thus, what is needed is a certificate meter that provides the user with a capability to selectively apply one of a plurality of digital signatures of varying levels of security to a specific message. The selected digital signature will have a validity period that is commensurate with the type of message being processed.
Furthermore, the certificate meter of the '841 patent provides an efficient mechanism for paying a certificate authority for its services but does not address the question of the monetary indemnification that the certificate authority is willing to provide to a user who suffers a loss based on placing trust in the security that was to be provided by using the signed message. The issue of indemnification is critical to all of the parties involved in the messaging transaction. That is, the receiver of the message wants assurance that he will be indemnified if he acts based on trust in the signed message and still suffers a monetary loss. Furthermore, assuming that the receiver has a notion of what loss he might potentially incur, then he specifically would desire an indemnification that covers such loss. On the other hand, the sender of the signed message wants the receiver to take an action based on the message contents and therefore would like to provide to the receiver proof that an appropriate level of indemnification is guaranteed by the digitally signed message. Finally, the certificate authority wants to receive a payment from the sender that varies according to the level of indemnification to be provided. Thus, what is needed is a certificate meter which permits the user the ability to select a specific level of indemnification for each digitally signed message he sends.